Conventionally, a friction stir welding apparatus including a probe-type rotary tool is known as a friction stir welding apparatus for joining together workpieces abutting each other. The probe of the probe-type rotary tool protrudes from an end face of a cylindrical body of the rotary tool. The rotary tool is, while being rotated, moved along a boundary between the workpieces in a state where the axial direction of the rotary tool is inclined relative to the workpieces, i.e., in such a manner that a part of the body of the rotary tool is inserted into the workpieces. As a result, a recess is formed on the front surface side of a weld joint.
In recent years, a bobbin-type rotary tool including a pair of shoulders for holding workpieces in a sandwiching manner, the shoulders being provided on both sides of a probe, respectively, has been developed. The bobbin-type rotary tool is, while being rotated, moved along a boundary between the workpieces in a state where the axial direction of the rotary tool is perpendicular to the workpieces.
For example, Patent Literature 1 discloses a friction stir welding apparatus capable of causing a first shoulder and a second shoulder to rotate at different rotation speeds. The first shoulder rotates on front surfaces of workpieces, and the second shoulder rotates on back surfaces of the workpieces.